Low Drop-Out (LDO) linear voltage regulator integrated circuits are widely used in electronic systems, particularly in applications which require power supplies with low noise and low ripple. In portable applications, LDO regulators supply power to the analog baseband stages, radio frequency stages and to other noise-sensitive analog circuit blocks.
The efficiency and the physical size of the power supply solution are two important aspects in portable applications where the amount of energy stored in the battery is limited and board space is at a premium. The efficiency loss of an LDO regulator has two principal components, namely thermal dissipation, and ground current.
FIG. 1 is a block diagram of an LDO regulator 10, as known in the prior art. LDO regulator 10 includes a pair of closed-loop amplifiers 12 and 14, and a PMOS pass transistor 16. Thermal dissipation is determined by the difference between the input and output voltages of the LDO regulator 10, and the current through PMOS transistor 16 which nearly equals the load current. When the difference between the input and the output voltages is large and high currents are delivered to the load, a large amount of power is dissipated by transistor 16. Minimizing the input-output voltage differential minimizes the energy so wasted for a given load current. Thus it is advantageous to operate the LDO regulators at a low input-output voltage differential. Minimizing the input-output voltage differential would require lowering of the supply voltage of the internal circuitry in LDO regulator 10 to a level close to the regulated output voltage, which in turn, poses a significant design challenge as lower output voltages are demanded from the output of the LDO regulator 10.
The ground current of an LDO regulator mostly includes bias currents for biasing of internal circuitry and for generating reference voltages and currents. The ground current does not contribute to the load current as it flows from the input supply to ground, through internal circuitry. Although at low load currents, stable LDO regulator operation can be achieved using relatively low bias currents, high load currents usually require high bias currents to ensure stable operation while ensuring good transient response. Conventional LDO regulators, such as that shown in FIG. 1, employ a constant biasing scheme with high internal bias currents to provide a stable operation at high load currents. Such a biasing scheme wastes valuable current at light loads and the light load efficiency suffers as a result. Another conventional approach is to keep the bias currents constant at a low to moderate level, but such an approach deteriorates the transient response of the LDO regulator.